1. Field of the Invention
The present invention is generally directed to cross-linked toner resins exhibiting improved xerographic properties, a process for making such cross-linked toner resins, and toners derived therefrom. More specifically, the present invention relates to cross-linked polyester resins such as propoxylated bisphenol A fumarate resins that are cross-linked in a reactive extrusion process in the presence of the liquid 1,1-bis(t-butyl peroxy)-3,3,5-trimethylcyclohexane as a chemical initiator.
2. Discussion of Related Art
Many processes are known for effecting polymerization reactions, including reactive extrusion processes, for both initial polymerization reactions employing monomers or prepolymers, and for polymer modification reactions, such as graft, coupling, cross-linking and degradation reactions.
U.S. Pat. No. 4,894,308 and U.S. Pat. No. 4,973,439, for example, disclose extrusion processes for preparing electrophotographic toner compositions in which pigment and charge control additive were dispersed into the binder resin in the extruder. However, in each of these patents, there is no suggestion of a chemical reaction occurring during extrusion, i.e., they do not relate to reactive extrusion processes for forming the toner resin.
An injection molding process for producing cross-linked synthetic resin molded articles is disclosed in U.S. Pat. No. 3,876,736 in which polyolefin or polyvinyl chloride resin and cross-linked agent were mixed in an extruder, and then introduced into an externally heated reaction chamber outside the extruder wherein the cross-linking reaction occurred at increased temperature and pressure, and at low or zero shear. In U.S. Pat. No. 4,089,917, an injection molding and cross-linking process is disclosed in which polyethylene resin and cross-linking agent were mixed in an extruder and reacted in reaction chambers at elevated temperature and pressure. Heating of the resin mixture occurred partially by high shear in inlet flow orifices. However, the cross-linking reaction in this process still took place in the reaction chambers at low or zero shear, and the final product is a thermoset molded part, and thus is not useful for toner resins.
A process for dispensing premixed reactive precursor polymer mixtures through a die for the purposes of reaction injection molding or coating is described in U.S. Pat. No. 4,990,293 in which polyurethane precursor systems were cross-linked in the die and not in the extruder. The dimensions of the die channel were determined such that the value of the wall shear stress is greater than a critical value in order to prevent gel buildup and consequent plugging of the die. The final product is a thermoset molded part, and thus is not useful for toner resins.
It should be noted that the processes disclosed in U.S. Pat. Nos. 3,876,736, 4,089,917 and 4,990,293 are not reactive extrusion processes, because the cross-linking in each case occurs in a die or a mold, and not in an extruder, and the cross-linking takes place at low or zero shear. These processes are for producing engineering plastics such as thermoset materials which cannot be remelted once molded, and thus are not suitable for toner application.
U.S. Pat. Nos. 5,227,460, 5,352,556, 5,376,494, 5,395,723 and 5,401,602, each incorporated herein by reference in their entireties, describe a low melt toner resin with low minimum fix temperature and wide fusing latitude that contains a linear portion and a cross-linked portion containing high density cross-linked microgel particles, but substantially no low density cross-linked polymer. It is described that the resin may be formed by reactive melt mixing under high shear and high temperature of an unsaturated polyester resin such as a poly(propoxylated bisphenol A fumarate) in the presence of a chemical initiator that has been mixed into the polyester. These patents describe that the chemical initiator may be selected from a large list of possible initiators, with benzoyl peroxide being preferred. The initiator 1,1-bis(t-butyl peroxy)-3,3,5-trimethylcyclohexane is mentioned in passing as being within the large group of possible initiators. The patents also describe that the initiator, whether solid or liquid, is mixed with the polyester resin, for example in a rotary tumble blender or a batch mixer, although U.S. Pat. No. 5,395,723 indicates that the mixing also may take place within the extruder.
U.S. Pat. No. 6,063,827, incorporated herein by reference in its entirety, describes a process for the preparation of an unsaturated polyester which comprises (i) reacting an organic diol with a cyclic alkylene carbonate in the presence of a first catalyst to thereby form a polyalkoxy diol, (ii) optionally adding thereto a further amount of cyclic alkylene carbonate in the presence of a second catalyst, and (iii) subsequently polycondensing the resulting mixture with a dicarboxylic acid. The unsaturated polyester formed may then be further subjected to cross-linking with an initiator as in the patents described immediately above in order to form a toner resin.
To date, benzoyl peroxide has been used as the preferred chemical initiator in the reactive extrusion methods of cross-linking unsaturated polyester resins. Although the cross-linked resins obtained by such processes exhibit, at least initially, satisfactory xerographic performance properties, several problems exist with respect to the use of benzoyl peroxide as an initiator.
First, when used as an initiator in the cross-linking of the unsaturated polyester resin, benzoic acid is produced as a by-product. Removal of benzoic acid from the formed cross-linked polyester is problematic in that although it can be partially removed as a volatile through a vacuum stripping procedure, the benzoic acid quickly condenses within the vacuum system and rapidly clogs the system, thereby causing frequent shutdowns for cleaning. As a result, residual benzoic acid, for example on the order of 0.25% by weight of the resin, typically remains in the product resin.
Second, benzoyl peroxide is a solid initiator that must be mixed with the unsaturated polyester, which is in granular form, as discussed above. As this physical blend is fed into the reactive extruder, flow-induced granule separation can and does occur. This produces a variability of the initiator concentration with time into the extruder, and thus a high variability of gel content with time in the cross-linked product.
Third, benzoyl peroxide is a hazardous chemical. The hydrated form of benzoyl peroxide dries out quickly and becomes a shock hazard after several hours.
Fourth, toners prepared based upon cross-linked resins in which benzoyl peroxide is used as the initiator are very sensitive to environmental conditions, and the xerographic performance of such toners decrease with increasing temperature and humidity conditions. The deteriorating performance properties include charging, humidity sensitivity of charging, and background density.
What is still desired is an improved cross-linked toner resin from unsaturated polyester resin and a process for making the same in which the foregoing shortcomings associated with the use of benzoyl peroxide as an initiator in forming the cross-linked toner resins are overcome.
It is an object of the present invention to develop a toner resin that possesses improved xerographic performance, particularly in terms of charging, humidity sensitivity and background density properties, compared to prior toner resins.
It is still a further object of the present invention to replace benzoyl peroxide as an initiator in the cross-linking of unsaturated polyester resin.
It is a still further object of the present invention to develop an improved process of cross-linking the unsaturated polyester resin so as to derive a cross-linked product of more uniform structure and properties.
These and other objects of the present invention are achieved herein.
The present invention thus relates to a cross-linked toner resin consisting essentially of linear portions and cross-linked portions, wherein the toner resin is an unsaturated polyester resin, wherein the cross-linked portions consist essentially of high density cross-linked microgel particles, and wherein the cross-linked toner resin contains less than 0.20 percent by weight of acids. In particular, the cross-linked toner resin is free of benzoic acid.
The present invention also relates to a method of making the cross-linked toner resin by (a) spraying a liquid chemical initiator onto the unsaturated polyester resin while blending and prior to, during or subsequent to melting of the unsaturated polyester resin to form a polymer melt; and (b) subsequently cross-linking the polymer melt under high shear to form the cross-linked toner resin.
Use of a liquid chemical initiator, mainly 1,1-bis(t-butyl peroxy)-3,3,5-trimethylcyclohexane, permits the initiator to be sprayed onto the unsaturated polyester resin, thereby permitting more uniform distribution of the initiator throughout the resin and thus more uniform cross-linking of the resin. Further, 1,1-bis(t-butyl peroxy)-3,3,5-trimethylcyclohexane enables a cross-linked resin to be obtained that exhibits better performance than cross-linked resins formed using benzoyl peroxide while avoiding the problems associated with the use of benzoyl peroxide, including avoiding the presence of residual by-product benzoic acid in the cross-linked resin.